The major goals are to examine the extent of plasticity in regulatory mechanisms for pituitary opiate peptide secretion, both in young rats, and in aging animals. The major hypothesis to be tested is that innervation to pro-opiomelanocortin (POMC) cells of the pituitary intermediate lobe retains the ability to regenerate and regulate peptide secretion after denervation in young individuals, and that during aging, there is modulation of secretion as both nerves and endocrine cells undergo potential degenerative alterations. Questions are: 1. How do POMC endocrine cells respond to denervation and to potential re-innervation over time? Young adult male rats will be treated with a catecholamine neurotoxin, 6-hydroxydopamine (6-OHDA_, which induces neurite degeneration in pituitary. Immunoreactive catecholamine and serotonin innervation will be studied in drug treated and control animals over periods of 8 days, 3 and 6-8 weeks after treatment, to determine the extent of denervation, and of potential re-innervation of endocrine cells. 2. Do POMC cells modify their receptor population to adapt to denervation and potential re- innervation? Pituitary tissue of 6-OHDA treated rats will be studied using autoradiographic techniques with specific radioligands which bind to dopamine-2 (D-2) and serotonin-2 (5-HT-2) receptors, for alterations in receptor patterns which may occur with denervation and re-innervation. 3. How do POMC cells respond to specific regulatory molecules after denervation, and during potential re-innervation? Pituitaries of 6-OHDA treated animals will be incubated in vitro to examine direct effects of inhibitory or stimulatory molecules on peptides secretion. Peptides will be measured by radioimmunoassay (RIA) and tissues studied by immunocytochemical and electron microscopic (EM) techniques. 4. What changes occur in POMC secretory cells and their innervation during normal aging of individual animals? Pituitaries of aging rats (6, 12 and 18 months) will be examined for potential alterations in innervation patterns as well as for modulation of peptide secretion, using immunocytochemistry and RIA for tissue or blood samples. EM techniques will be used to correlate cytology of aging pituitaries with alteration in immunoreactive peptides. To determine if D-2 and 5-HT-2 receptors are altered with age, autoradiographic patterns of ligand binding will be examined. 5. Do aging POMC cells retain the ability to respond to regulatory molecules which inhibit or stimulate secretion? Pituitaries from aging rats will be studied in vitro using neurotransmitters and peptides which have been found to modulate opiate peptide release in young animals. Opiate peptides have significant effects on regulation of pain and on homeostatic mechanisms related to stress. Investigations of plasticity of regulatory mechanisms, in the forms of potential neurite regeneration and modulation of receptor patterns on POMC cells, both in young and aging animals, will be a basis for appropriate pharmacologic manipulation of opiate peptide secretion, in animal models, and in man.